Method for the production of very low sulfur diesel

ABSTRACT

A process for the deep desulfurization of diesel range feedstock to produce low sulfur diesel fuels by contacting a sulfur containing diesel range feedstock with a cobalt molybdenum (CoMo) catalyst followed by a nickel containing catalyst, such as nickel molybdenum (NiMo), nickel tungsten (NiW), nickel tungsten molybdenum (NiWMo) and nickel cobalt molybdenum (NiCoMo), under a combination of elevated temperature and superatmospheric hydrogen pressure to convert the sulfur in the sulfur-containing feedstock to inorganic sulfur compounds and produce a desulfurized product having a sulfur content below SO ppm by weight. The process can include a dual catalyst system, wherein the sulfur containing diesel range feedstock is desulfurized with a cobalt molybdenum (CoMo) catalyst and then the sulfur compounds can optionally be stripped from the stream prior to contacting with the nickel containing catalyst. The preferred desulfurized product contains less than 11 wt. % polycyclicaromatics and has an increased cetane number.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application is a continuation-in-part of U.S. patentapplication Ser. No. 09/431,423 filed Nov. 1, 1999.

BACKGROUND OF INVENTION

[0002] The present invention relates to the desulfurization of dieselfuels, and, in particular, to reducing the sulfur content of dieselfuels to a very low level, while also yielding a high cetane value.

[0003] The sulfur impurities in diesel fuels require removal, usually byhydrotreating, in order to comply with product specifications and/or toensure compliance with environmental regulations. The current U.S.specification for diesel fuels permits a maximum sulfur content of 50ppmw. However, the EPA is expected to propose new diesel fuelspecifications that will become effective in 2004. The new specificationis likely to require further reduction of sulfur content in diesel fuelsto below 50 ppmw. Recently, the European Union published new dieselspecifications, which limit the sulfur content of diesel fuels to amaximum of 350 ppmw after the year 2000, and to 50 ppmw maximum afterthe year 2004. In addition, new specifications have been proposed whichwill increase the cetane number of diesel fuels to 58 in the year 2005,and reduce the polyaromatics content.

[0004] With the enactment of stricter diesel specifications, theprocesses presently being used for producing diesel fuels may notsufficiently reduce the sulfur content. This will require a modificationof existing processes or the introduction of new processes. However,many of the new diesel specifications also require a higher cetane valueand so any modified or new desulfurization process will also have todecrease the sulfur content while maintaining or increasing the cetanevalue. Diesel fuels can be hydrotreated by passing the feed over ahydrotreating catalyst at an elevated temperature and a somewhatelevated pressure in a hydrogen atmosphere. One suitable family ofcatalysts which has been widely used for this service is a combinationof a Group VIII metal and a Group VI metal of the Periodic Table, suchas cobalt and molybdenum, on a substrate such as alumina. After thehydrotreating operation is complete, the product can be fractionated, orsimply flashed, to release the hydrogen sulfide, remove low flash lightgases and collect the sweetened diesel fuel.

[0005] Various proposals have been made for removing sulfur whileretaining the more desirable paraffinic components. For example, U.S.Pat. No. 3,546,103 teaches hydrodesulfurization with a catalyst ofcobalt and molybdenum on an alumina base.

[0006] Although the art of hydroprocessing has been known for a longtime and is a highly developed art, there exists today even greater needfor efficient and economical means for hydrodesulfurizing diesel fuelsin order to comply with more stringent diesel fuel specifications andenvironmental regulations. Therefore, a process for the deepdesulfurization of diesel fuels that will also maintain a high cetanevalue, or increase the cetane value, is essential to the upgrading ofsuch stocks. None of the prior art mentioned above nor any prior artknown to applicant discloses a catalyst which is capable of efficientlyhydrodesulfurizing diesel fuels while providing an improved cetane valueand polyaromatics saturation.

SUMMARY OF THE INVENTION

[0007] In accordance with the present invention, a process is providedfor the desulfurization of diesel boiling range feedstocks to producelow sulfur diesel fuels. The process includes contacting a sulfurcontaining diesel boiling range feedstock with a cobalt molybdenum(CoMo)-containing catalyst, followed by a nickel (Ni)-containingcatalyst under a combination of elevated temperature andsuperatmospheric hydrogen pressure to convert the sulfur in thesulfur-containing feedstock to inorganic sulfur compounds and produce adesulfurized product having a sulfur content below 50 ppm by weight(ppmw). The feedstock can be desulfurized prior to contacting with thenickel containing catalyst or a desulfurized diesel fuel containing lessthan 0.2 wt. % can be used as the feedstock. The nickel-containingcatalyst includes a desulfurization component of nickel molybdenum(NiMo), nickel tungsten (NiW), nickel tungsten molybdenum (NiWMo) ornickel cobalt molybdenum (NiCoMo); and a support of alumina,silica-alumina, titania, magnesia, zirconia, silica, zeolite,non-zeolite molecular sieve or combinations thereof.

[0008] In one embodiment, the process is carried out in at least twostages, wherein the feedstock is desulfurized in a first stage beforebeing contacted with a nickel containing catalyst to achieve deepdesulfurization in a second stage. In a preferred embodiment, theprocess is carried out using a dual catalyst system, wherein thefeedstock is contacted with a cobalt molybdenum/alumina (CoMo/Al₂O₃)catalyst in the first stage to produce a desulfurized feedstock prior tocontacting the feedstock with the nickel containing catalyst. After thefeedstock is desulfurized in the first stage, the feedstock can bestripped of sulfur and, nitrogen compounds (primarily H₂S and NH₃)before it is contacted with the nickel containing catalyst in the secondstage. The dual catalyst system process can be carried out in onereactor vessel or in multiple vessels. The feedstock can be desulfurizedby a CoMo catalyst system in a first reactor, and then sent to astripper, before being contacted with a nickel containing catalyst,preferably NiMo/Al₂O₃, in the second reactor.

[0009] In addition to desulfurizing the feedstock, the process improvesthe cetane number so that the cetane number of the product is at leastequal to, and in preferred embodiments greater than, the cetane numberof the diesel range feedstock. In preferred embodiments, the processreduces the polyaromatics in the desulfurized product to less than 11wt. %.

[0010] The process of the present invention provides deepdesulfurization of diesel range feedstocks by using nickel containinghydrotreating catalysts to remove the last refractory sulfur compoundsfrom diesel fuels. This process significantly reduces the necessaryvolume of the reactor and provides substantial savings due to the highactivity of the nickel containing catalysts compared to conventionalCoMo catalysts. The increased activity of these catalysts requiressmaller reactors and, thus, can save as much as 50-70% of the catalystvolume compared to processes that use CoMo catalysts.

BRIEF DESCRIPTION OF THE FIGURES

[0011] Other objects and many attendant features of this invention willbe readily appreciated as the invention becomes better understood byreference to the following detailed description when considered inconnection with the accompanying drawings wherein:

[0012]FIG. 1 is a graph showing sulfur reduction using a CoMo catalystand a NiMo catalyst.

[0013]FIG. 2 is a graph showing polyaromatics saturation using a CoMocatalyst and a NiMo catalyst.

[0014]FIG. 3 is graph showing the cetane number for desulfurized productusing a CoMo catalyst and a NiMo catalyst.

[0015]FIG. 4 is a flow diagram of a preferred embodiment of the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

[0016] Diesel fuels currently being used will require additionaldesulfurization in order to meet the stricter government regulationsthat are expected to be enacted in the next few years. The presentinvention provides a method for desulfurizing diesel fuel feedstocks tobelow 50 ppmw (parts per million by weight) sulfur to comply with theseregulations by using a nickel containing catalyst to desulfurizerefractory sulfur-containing components of the feedstocks. The processcan also be used to provide deep distillate desulfurization of dieselfuels currently being produced that do not comply with the stricterregulations.

[0017] Most desulfurization processes for diesel fuel feedstocks usecobalt molybdenum (CoMo) based catalysts instead of nickel basedcatalysts because CoMo based catalysts are more active when treatingfeedstocks with a relatively high sulfur content. The present inventionis based on the discovery that NiMo catalysts are more active than CoMo,catalysts for deep desulfurization of diesel fuels having, relativelylow sulfur levels. Using the method, of the present invention, dieselfuels, containing low sulfur levels are desulfurized to produce dieselproducts containing <50 ppm sulfur. In addition, NiMo catalysts are moreeffective than CoMo catalysts in improving the cetane number andpolyaromatics saturation.

[0018] In a preferred embodiment of the present invention, a NiMo/Al₂O₃catalyst is used for post desulfurization of current 0.05 wt. % S dieselto less than 50 ppmw sulfur. In another embodiment, a dual catalystsystem consisting of a CoMo catalyst followed by a nickel basedcatalyst, such as a NiMo catalyst, is used for desulfurization of rawdiesel range feedstocks. The CoMo/NiMo catalyst system has been found tobe more active for deep distillate desulfurization than a reverseNiMo/CoMo catalyst system, or a CoMo catalyst system alone. While CoMocatalysts are effective in reducing the sulfur level of raw diesel rangefeedstocks to below 0.2 wt. %, it has been found that NiMo catalysts aremore active for desulfurization of residual refractory sulfur-containingcompounds. CoMo catalysts are effective in desulfurizing most of theorganic sulfur compounds in diesel fuel feedstocks but, by comparison,have been found to be relatively ineffective in desulfurizing refractorysulfur-containing compounds that have at least one alkyl (primary methylgroup) substitute adjacent to the sulfur atom of dibenzothiophene typecompounds, such as 4,-methyl or 4,6-dimethyl dibenzothiophene and theiralkyl homologs. Conversely, nickel containing catalysts, which are notas active as CoMo catalysts for desulfurizing many organicsulfur-containing compounds, are particularly well suited fordesulfurizing refractory sulfur-containing compounds and have asignificantly higher activity when processing these compounds than CoMocatalysts.

[0019] Sulfur compounds that have at least one methyl group adjacent tothe sulfur atom, such as 4-methyl dibenzothiophene, 4,6dimethyldibenzothiophene and related compounds, are very difficult to remove ina desulfurization process. Generally, these are the primary sulfurcompounds that remain in diesel fuels desulfurized to less than 0.2 wt.% sulfur using conventional CoMo catalyst based desulfurizationprocesses. These CoMo catalyst processes are marginally effective inreducing these refractory sulfur-containing compounds and, therefore,they usually require high temperatures and/or prolonged residence timesto efficiently reduce sulfur levels to below about 50 ppmw as requiredby the new regulations. The process of the present invention uses anickel containing catalysts, such as a NiMo catalyst, in a second “deepdesulfurization” stage to reduce sulfur levels to below 50 ppmw. Thenickel containing catalysts are more active than CoMo catalysts fordesulfurizing these refractory sulfur-containing compounds and can moreefficiently produce diesel fuels with low sulfur levels than the dieselfuels produced by known processes.

[0020] The deep desulfurization nickel containing catalysts can also beused as combination dual catalyst system, such as a CoMo/NiMo dualcatalyst system, for desulfurizing raw diesel range feedstocks. In thefirst stage of the process, raw diesel range feedstocks are desulfurizedto below 0.2 wt. % sulfur using a CoMo catalyst and then the secondstage uses a nickel containing catalyst to desulfurize the feedstocks tobelow 50 ppmw sulfur. The dual CoMo/nickel containing catalyst processcan be carried out in separate reactors or in a single reactor. Forexample, in a downflow reactor, the top section of the catalyst bed cancontain CoMo based catalyst and the bottom section can contain NiMocatalyst. The method of operating this type of dual bed reactor is wellknown to those skilled in the art. The diesel fuel product will have aboiling point range of about 350° F. to about 650° F. (about 175° C. toabout 345° C.). The process of the invention can be used to upgrade afeedstock within the diesel fuel boiling point range to a higher cetanediesel fuel.

[0021] Cetane number is calculated by using either the standard ASTMengine test or NMR analysis. Although cetane number and cetane indexhave both been used in the past as measures of the ignition, quality ofdiesel fuels, they should not be used interchangeably. Cetane index canfrequently overestimate the quality of diesel fuel streams containingcracked stocks. Thus, cetane number is used herein.

[0022] The diesel boiling range feedstocks product can generally bedescribed as high boiling feeds of petroleum origin. In general, suchfeedstocks include gas oils distilled from various petroleum sources,boiling point from about 350° F. to about 750° F. (about 175° C. toabout 400° C.), preferably about 400° F. to about 700° F. (about 205° C.to about 370° C.). Catalytic cracking cycle oils, including light catcycle oil (LCCO) and heavy cat cycle oil (HCCO), clarified slurry oil(CSO), other catalytically cracked products, and thermally crackedproducts, such as coker light gas oil, are potential sources of feedsfor the present process. If used, it is preferred that these cycle oilsmake up a minor component of the feed. Cycle oils from catalytic andthermal cracking processes typically have a boiling range of about 400°F. to 750° F. (about 205° C. to 400° C.), although light cycle oils mayhave a lower end point, e.g. 600° F. or 650° F. (about 315° C. or 345°C.). Because of the high content of aromatics found in such cycle oils,as well as undesirable amounts of nitrogen and sulfur, they require moresevere process conditions. Lighter feeds may also be used, e.g. about250° F. to about 400° F. (about 120° C. to about 205° C.). However, theuse of lighter feeds will result in the production of higher value,lighter distillate products, such as kerosene.

[0023] The feed to the process can be rich in naphthenic species, suchas found in a hydrocrackate product. The naphthenic content of the feedsused in the present process generally will be at least 5 wt. %, usuallyat least 20 wt. %, and in many cases at least 50 wt. %. The balance willbe divided among paraffins and aromatics according to the origin of thefeed and its previous processing.

[0024] The nickel containing catalyst stage of the process operates witha relatively low sulfur feed, generally less than about 1.0 wt. % sulfurby weight and preferably less than 0.2 wt. %. Hydrotreated or hydrocracked feeds are preferred, because both processes remove sulfur andnitrogen compounds from hydrocarbon streams without substantial boilingrange conversion. In addition, for some feeds hydrotreating saturatesaromatics to naphthenes, and hydrocracking produces distillate streamsrich in naphthenic species.

[0025] In a preferred embodiment, the selected sulfur-containing, dieselboiling range feed is desulfurized in two stages. In the first stage,sulfur compounds present in the feed are converted to the inorganic form(i.e., H₂S) in the presence of a CoMo catalyst. In the second stage,desulfurization is carried out in the presence of a nickel containingcatalyst. In one embodiment, the inorganic compounds are separated fromthe diesel fuel stream between the first and second stage. However, inanother embodiment the first stage effluent can be cascaded directlyinto the second stage reactor without the need for interstageseparation.

[0026] When a two stage process is used, the particle size and thenature of the catalysts used in both stages will usually be determinedby the type of process used, such as: a down-flow, liquid phase, fixedbed trickle flow process; an up-flow, fixed bed countercurrent process;an ebullating, fluidized bed process; or a transport, fluidized bedprocess. All of these different process schemes, which are well known,are possible although the down-flow fixed bed arrangement has theadvantage of simplicity of operation.

[0027] The present invention can be operated as a single stage process,wherein a diesel fuel feedstock having a sulfur level of less than 1.0wt. %, preferably less than 0.2 wt. %, is contacted with a nickelcontaining catalyst to reduce the sulfur content to below 50 ppmw. Forfeedstocks with a higher sulfur content, a preferred embodiment of theinvention uses a two stage process, wherein the feedstock isdesulfurized in two stages. The following description is for a two stageprocess. When a one stage process is used, the first stage of the twostage process is bypassed and the feedstock is desulfurized inaccordance with the description for the second stage processing.

[0028] First Stage Processing

[0029] The first stage of the process desulfurizes the diesel fuelfeedstock using any one of several well known hydrodesulfurizationprocesses, preferably but not necessarily a process which employs a CoMocatalyst. Similar processes are disclosed in U.S. Pat. No. 4,568,448Angevine et al., U.S. Pat. No. 5,011,593 Ware et al., U.S. Pat. No.5,401,389 Mazzone et al. and U.S. Pat. No. 5,865,988 Collins et al., allof which are incorporated by reference herein in their entirety.

[0030] The catalyst used in the first stage hydrodesulfurization can bea conventional desulfurization catalyst made up of a Group VI and/or aGroup VIII metal on a suitable substrate. The Group VI metal is usuallymolybdenum or tungsten and the Group VIII metal usually nickel orcobalt. Combinations such as Ni—Mo or Co—Mo are typical. Other metalswhich possess hydrogenation functionality are also useful in thisservice. The support for the catalyst is conventionally a porous solid,usually alumina, or silica-alumina but other porous solids such asmagnesia, titania or silica, either alone or mixed with alumina orsilica-alumina may also be used, as convenient.

[0031] The conditions used in the first stage of the process are thosewhich result in the controlled formation of inorganic sulfur compounds,such as H₂S. Typically, the temperature of the first stage reactor willbe from about 300° F. to 850° F. (about 150° C. to 455° C.), preferablyabout 350° F. to 800° F. (about 177° C. to about 427° C.). Since thedesulfurization of the diesel fuel feedstocks normally takes placereadily, low to moderate pressures can be used. The pressure, therefore,will depend mostly on operating convenience. Pressure will typically beabout 50 to 1500 psig (about 445 to 10445 kPa), preferably about 300 to1000 psig (about 2170 to 7000 kPa) with space velocities typically fromabout 0.5 to 10 LHSV (hr−¹), normally about 1 to 6 LHSV (hr−¹). Hydrogento hydrocarbon ratios typically of about 200 to 5000 SCF/Bbl (36 to 890n.1.1¹.), preferably about 500 to 2500 SCF/Bbl (about 89 to 445 n.1.1−1)will be selected to minimize catalyst aging.

[0032] Second Stage Processing

[0033] The feed to the second stage can be the first stage effluent orthe effluent from another desulfurized diesel boiling range stream andshould contain less than 1.0 wt. % sulfur by weight, and preferably lessthan 0.2 wt. %. Diesel fuels conforming to the current regulatorystandards have a sulfur content of less the 0.05 recent by weight andcan be used as the feed to the second stage of the process. In thesecond stage, the desulfurized diesel boiling range stream undergoesdeep desulfurization by contacting the stream with a nickel containingcatalyst, such as nickel molybdenum (NiMo), nickel tungsten (NiW),nickel tungsten molybdenum (NiWMo) and nickel cobalt molybdenum(NiCoMo).

[0034] After the first stage and before being sent to the second stageof the process, the desulfurized diesel boiling range effluent streamcan be stripped of H₂S, NH₃ and light gases, which were formed in thefirst desulfurization stage. Stripping the effluent of H₂S and NH₃ makesthe second stage desulfurization more efficient.

[0035] The temperature of the second hydrodesulfurization step is fromabout 400° F. to 850° F. (about 220° C. to 454° C.), preferably about500° F. to 750°F. (about 260° C. to 400° C.), with the exact selectiondependent on the desulfurization required for a given feed with thechosen catalyst. A temperature rise occurs under the exothermic reactionconditions, with values of about 20° F. to 100° F. (about 11° C. to 55°C.) being typical under most conditions and with reactor inlettemperatures in the preferred 500° F. to 750° F. (260° C. to 400° C.)range.

[0036] Since the desulfurization of the diesel boiling range effluentstream normally takes place readily, low to moderate pressures maybeused, typically from about 50 to 1500 psig (about 445 to 10443 kPa),preferably about 300 to 1000 psig (about 2170 to 7,000 kPa). Pressuresare total system pressure, reactor inlet. Pressure will normally bechosen to maintain the desired aging for the catalyst in use. The spacevelocity is typically about 0.3 to 10 LHSV (hr.−¹), preferably about 1to 6 LHSV (hr.−¹). The hydrogen to hydrocarbon ratio in the feed istypically about 500 to 5000 SCF/Bbl (about 90 to 900 n.1.1^(.−1)),usually about 1000 to 2500 SCF/B (about 180 to 445 n.1.1^(.−1)). Theextent of the desulfurization will depend on the feed sulfur content andon the product sulfur specification, with the reaction parametersselected accordingly. Normally, the process will be operated under acombination of conditions such that the second desulfurization stageyields a product having less than 50 ppm sulfur.

[0037] The catalyst used in the second hydrodesulfurization step issuitably a nickel containing desulfurization catalyst including one ormore Group VI and/or a Group VIII metals on a suitable substrate. TheGroup VI metal is usually molybdenum or tungsten and the Group VIIImetal usually nickel or cobalt. The preferred combinations are NiMo,NiW, NiWMo and NiCoMo. The support for the catalyst is conventionally aporous refractory metal oxide, such as alumina or silica-alumina butother porous solids such as magnesia, zirconia, titania or silica,either alone or mixed with alumina or silica-alumina may also be used.Under certain conditions, it may be desirable to include a zeolite orother molecular sieve in the support to provide improved dispersion ofthe hydrogenation component or enhanced resistance to poisons, such asH₂S. Other metals which possess hydrogenation functionality are alsouseful in this service. In a preferred embodiment, a high hydrogenationcatalyst is combined with the NiMo catalyst to provide maximumdesulfurization. The high hydrogenation activity catalysts include Pt orPt/Pd promoted alumina or zeolite containing catalysts.

[0038] The particle size and the nature of the catalyst will usually bedetermined by the type of conversion process which is being carried out,such as: a trickle flow fixed bed process; a counter-current (liquidphase downflow, gas phase upflow) fixed bed process, or an ebulatted,bed process, as noted above, with the down-flow fixed-bed trickle flowtype of operation typically preferred.

EXAMPLE 1

[0039] Two commercial catalysts (NiMo/Al₂O₃ and CoMo/Al₂O₃) were used atthe same conditions to desulfurize a partially desulfurized distillatestream with sulfur-compounds that contained at least one methyl group onthe aromatic ring adjacent to the sulfur atom. The feedstock was a 0.05wt. % sulfur diesel fuel obtained from a commercial desulfurization unitrundown (Table 1). This feed contained 429 ppmw sulfur associated withthe refractory sulfur compounds. Both catalysts were tested separatelyin a fixed bed, down flow, trickle-bed reactor at the same conditions,i.e., 288-370° C., 2 LHSV, 42.2 kg/cm² G total pressure and 2670 Nm³/m³hydrogen circulation rate. TABLE 1 DESULFURIZED DIESEL WITH 0.05 WT. %SULFUR PARAMETER VALUE Gravity, ° API 31.8 Nitrogen, ppmw 230 CetaneIndex 38.7 Sulfur, ppmw 4-methyl DBT^((a)) 6 4,6-dimethyl DBT 122 3+Carbon DBT^((b)) 301 2,3-dimethyl DBT 23 Total sulfur 452 Totalrefractory sulfur, ppmw 429 Aromatic Content, wt. % Monoaromatics 29.1Polyaromatics 13.8 Total aromatics 42.9 Distillation (D86), ° F. IBP 34610% 411 50% 494 90% 606 EBP 668

[0040] The desulfurization results are shown in FIG. 1 and clearly showthat the NiMo catalyst is more active for desulfurization <50 ppmwsulfur. At reaction temperatures between 325° C. and 375° C. (the rangewithin which the sulfur content is below 50 ppmw for the NiMo catalyst),the NiMo catalyst reduces the sulfur level of the diesel fuel from 50 to125 ppmw more than the conventional CoMo catalyst. The activityadvantage of the NiMo catalyst over the CoMo catalyst can be translatedto 50-70% catalyst volume saving. Because less NiMo catalyst is requiredto provide the same activity as the CoMo catalyst currently being used,the catalyst cost is reduced and a smaller reactor vessel can be used.

[0041] The higher activity of the NiMo catalyst not only resulted inmore efficient desulfurization, but also was more active for saturationof polycyclic aromatics, which is an important catalytic function tomeet the new government regulations for diesel fuels. The new Europeandiesel fuel specifications are expected to limit the polycyclicaromaticcontent of diesel fuels to less than 11 wt. %. Therefore, it isdesirable for a catalytic desulfurization process to also provide somepolycyclicaromatic saturation. FIG. 2 shows a comparison of thepolycyclicaromatics saturation for the diesel fuel products treated withthe NiMo catalyst and the CoMo catalyst. Between 325° C. and 360° C.(the range within which the sulfur content is below 50 ppmw),desulfurization with the NiMo catalyst produces a product withpolyaromatics content lower than the CoMo product and significantlybelow the expected regulatory limit of 11 wt. %. At the highertemperatures (e.g., 360° C. and above), the catalysts' performances areroughly equivalent as they are limited by thermodynamic equilibrium.

[0042] Aromatics saturation generally increases the cetane number of adiesel fuel. Consequently, the NiMo catalyst with its betterpolycyclicaromatics saturation produces a desulfurized diesel fuelproduct with a higher cetane number than the CoMo catalyst. FIG. 3 is agraph which compares the cetane number of desulfurized diesel fuelproducts produced using NiMo and CoMo over temperatures between 275° C.and 375° C. Between 325° C. and 360° C. (the range within which thesulfur content is below 50 ppmw), the cetane number for the NiMocatalyst products is higher than the cetane number for the CoMo catalystproducts.

[0043] A simplified flow diagram (which omits ancillary equipment) for apreferred embodiment of the present invention is shown in FIG. 4. Inthis embodiment, the effluent from the first desulfurization stage(using a CoMo catalyst) is sent to a stripper, which removes H₂S, NH₃and light hydrocarbons (e.g., methane, ethane, propane and butanes),before the effluent is fed into the second stage reactor where it iscontacted with a nickel containing catalyst (NiMo) to produce a dieselfuel product containing less than 50 ppmw sulfur.

EXAMPLE 2

[0044] This example compares the desulfurization activity of NiMo andCoMo catalyst systems in two different loading configurations using adiesel fuel containing refractory sulfur compounds. The tests wereconducted in a fixed-bed, down-flow trickle-bed pilot unit. For thefirst test, the reactor was loaded with a CoMo catalyst in the topsection of the reactor and NiMo catalyst in the bottom section in a50/50 volumetric ratio. For the second test, the catalyst loadingsequence was reversed, with a CoMo catalyst in the bottom section andNiMo catalyst in the top section. Both tests were carried out under thesame conditions, using the same raw distillate as the feedstock (Table2) for each test. The results listed in Table 3 show that the ACoMo/NiMo catalyst configuration was more active than the NiMo/CoMocatalyst configuration for the desulfurization of the diesel fuel. Theseresults indicate that the CoMo catalyst in the top section is moreactive for the initial desulfurization than the catalyst system usingthe NiMo catalyst and the NiMo catalyst system is more active for thedeep desulfurization of the refractory sulfur compounds. TABLE 2 RAWDIESEL DISTILLATE PARAMETER VALUE Gravity, ° API 28.0 Nitrogen, ppmw 250Total Sulfur, wt. % 2.0 Aromatic Content, wt. % Monoaromatics 19.7Polyaromatics 31.4 Total aromatics 51.1 Distillation (D86), ° F. IBP 31610% 415 50% 506 90% 605 EBP 642

[0045] TABLE 3 DUAL CATALYST CONFIGURATION PERFORMANCE DUAL CATALYSTCoMo/NiMo NiMo/CoMo Temperature, ° C. 342   370 343 370 Desulfurization,wt. %  86.5  95  84 93.5 I

[0046] Operating conditions: 3 LHSV, 35.2 kg/cm² G pressure, 178 Nm³/m³H₂ circulation.

EXAMPLE 3

[0047] This experiment shows that other nickel containing catalysts (inaddition to the NiMo catalyst), such as NiCoMo catalysts (and byextension, also NiW and NiWMo catalysts) can be successfully used todesulfurize refractory sulfur compounds. In this example a NiCoMocatalyst was compared with a NiMo catalyst for desulfurizing a rawdiesel distillate that contains 1.38 wt. % sulfur. The test results inTable 4 show the NiCoMo catalyst is more active than the CoMo catalystin attaining less than 50 ppmw. The NiCoMo attained 43 ppmw sulfur at366° C., while the CoMo only attained 56 ppmw sulfur at the sametemperature. TABLE 4 NiCoMo CATALYST PERFORMANCE NiCoMo NiMo CatalystTemperature, ° C. Residual Sulfur, wt. % Residual Sulfur, wt. % 2990.48  0.48  316 0.21  0.28  366 0.043 0.056 378 0.006 0.006

[0048] Thus, while there have been described the preferred embodimentsof the present invention, those skilled in the art will realize thatother embodiments can be made without departing from the spirit of theinvention, and it is intended to include all such further modificationsand changes as come within the true scope of the claims set forthherein.

1. A process for the desulfurization of diesel boiling range feedstocksto produce low sulfur, low polynuclear diesel fuels comprising:contacting a sulfur-containing diesel boiling range feedstock with arefractory supported cobalt molybdenum containing catalyst, thenstripping said feedstock, followed by a second stage containing arefractory supported nickel cobalt molybdenum catalyst under acombination of elevated temperature and superatmospheric hydrogenpressure, to convert said sulfur in said sulfur-containing feedstock toinorganic sulfur compounds and produce a desulfurized product having asulfur content below 50 ppm by weight, wherein (i) the first stage'stemperature ranges from about 150° C. to about 455° C. and the firststage's pressure ranges from about 50 to 1500 psig, and (ii) the secondstage's temperature ranges from about 220° C. to about 454° C. and thesecond stage's pressure ranges from about 50 psig to about 1500 psig. 2.The process of claim 1, wherein said process is carried out in onereactor vessel.
 3. The process of claim 1, wherein said diesel boilingrange feedstock has a first cetane number and said desulfurized producthas a second cetane number, and wherein said second cetane number isgreater than said first cetane number.
 4. The process of claim 1,wherein said desulfurized product contains less than 11 wt. %polyaromatics.
 5. The process of claim 1, wherein said diesel rangefeedstock contains less than 0.05 wt. % sulfur.
 6. The process of claim3, wherein said diesel range feedstock has a polycyclicaromatics contentof less than 11 wt. %.
 7. The process of claim 5, wherein saiddesulfurized product contains less than 11 wt. % polycyclicaromatics.